Infrared absorption spectrometry is a well-known technique for the assay of materials, and its use in combination with other analytical techniques is widely practiced. A combined assay finding wide application is the spectrometric assay of the constituents of a mixture, the mixture having first been divided into its constituent parts by gas chromatography.
While in principle the various fractions of a mixture separated by gas chromatography could be individually trapped (and concentrated) for subsequent spectrometric analysis, such a procedure is often inconvenient. Alternative approaches include slowing down or intermittently stopping the flow through the chromatographic column sufficiently to allow a fraction to be observed spectrometrically. Either of these alternative approaches is also frequently inconvenient, and further, leads to degraded chromatographic resolution (this last disadvantage is particularly true in the case of intermittently stopping the flow through the column).
Another approach, and one which is finding increasing application, is to make the spectrometric observations of the fractions "on the fly" as each fraction is eluted from the column at the normal carrier gas velocity. In this approach, the gas flowing from the chromatographic column is made to flow continuously through an optical cell forming a portion of the optical path of an illumination source/spectrometer system. The exhaust from the optical cell may in turn be throttled to atmosphere or conveyed to a further analysis station, as, for instance, a flame ionization detector or a mass spectrograph. It is with regard to such apparatus that the present invention is concerned.
It will be appreciated that the optical cell used for "on the fly" analysis must not only be matached to the optical requirements of the spectrometer, but to the flow requirements of the chromatograph (and any subsequent detectors) as well. The optical cell should present sufficient optical path length through the sample to insure adequate optical absorption and further be throughput-matched to the aperture and field of view of the source/spectrometer optical system. Chromatographically, the cell should not materially affect the flow rate of the chromatograph nor be a source of leakage or dead volume to any subsequent detectors.
The optical requirements for long path length and throughput matching of a cell delimiting a flowing stream have been met in prior art devices by flowing the sample through an elongate hollow tube with transparent ends, and observing the sample along the axis of the tube, the tube functioning as a light pipe. The desireable long path length is provided by the length of the tube, while multiple reflections from the interiorwalls of the tube act to preserve the system's optical throughput. For infrared spectrometry, as is commonly employed in such applications, the tube may be fabricated of any of a number of materials, and its inner surface made highly reflective as, for instance, by a gold coating. The windows of such cells must be highly transparent in the spectral region of interest, and, for the infrared, are often polished plates of sodium chloride. This latter material is both water soluble and easily damaged mechanically, and consequently such cells are fabricated with removable end windows, the mounting of the windows both sealing the cell and providing for differential expansion of the component parts of the cell. Typically, the end-window mounting also incorporates the couplings for the fluid transfer lines between the chromatograph and the cell and between the cell and subsequent devices.
Such sample cells have been successfully used for spectrometric assays of the effluent from normal gas chromatographic systems, where the sample size is sufficient to minimize the effect of the small dead volume inherent in the tube interior between windows. The situation is complicated in the case of capillary chromatography, however, where the sample is typically less than 10.sup.-7 gram and where concentrations of each of the eluted fractions are on the order of 10.sup.-8 g/ml. For such small samples and low fraction concentrations, leakage and dead volume are critical factors. From a spectrometric standpoint, the dead volume of prior art light-pipe flow cells is a significant percentage of the small size of the eluted sample fraction. Consequently, for such samples in such optical cells, spectrometric detection and chromatographic resolution are seriously degraded. The situation is further exacerbated by the short dwell time of an eluted fraction in the optical cell. While the outlet flow rate of a typical capillary column is generally on the order of 1 cc/min., the linear flow rate is considerable, due to the small bore (0.2-0.5 mm) of the column.
Additionally, the small sample size makes contamination or absorption of the sample by seals and the like a serious source of potential error.
An approach that has been employed to compensate in part for the optically non-observed dead volume in prior art optical cells has been to inject additional carrier gas into the stream flowing into the cell, thereby diluting the eluted fraction and making possible an overall larger volume cell with a correspondingly smaller percentage dead volume. However, this approach is not without its own disadvantages. While the increased cell volume permits a greater-length optical path, the length gain is offset by the dilution of the sample. Further, the maintenance of optical throughput in a longer cell requires more reflections from the cell wall, with a consequent loss in cell transmission and optical performance. It will also be recognized that the dilution of the fraction will adversely affect the performance of any concentration sensitive systems following the optical cell. Additionally, this approach does not address the problem of contamination or loss of sample through interaction with the various seals and packings used to secure the transfer lines, the windows, and the light pipe to one another.
Accordingly, it is an object of the present invention to provide an optical cell for the spectrometric analysis of small fluid samples having a minimum dead volume.
It is also an object of the present invention to provide such an optical cell which is particularly suited for "on the fly" analysis of fractions eluted from a capillary gas chromatography.
It is a further object of the present invention to provide such an optical cell which does not require the further dilution of an eluted fraction.
Yet again, it is an object of the present invention to provide an optical cell wherein the effects of the interaction between seals, packings, and sample are minimized.